Gibberellins are powerful plant hormones which are responsible for flowering, root growth, stem elongation, fruit size, branching and the like for various fruits and crops. The mixture of gibberellins GA4 and GA7 and pure GA3 are the only gibberellins presently commercially produced in quantity from cultures of the fungus Gibberella fujikoroi. These gibberellins are, therefore, convenient starting materials for the synthesis of less accessible gibberellins and, themselves, are powerful plant hormones which are important for use in agriculture.
There has been a long-standing need for a method which effectively separates GA4 and GA7 from the above mixture. Previously, tedious reverse-phase high performance liquid chromatography (HPLC) was used for separation of the mixture of GA4 and GA7. This process was labor intensive and not feasible for the preparation of large quantities. Some laboratory scale chemical processes have been used for the preparation of GA4 and GA7 in small quantities. However, these processes all involve multiple step syntheses. For example, as described in U.S. Pat. No. 4,243,594, GA7 is obtained from GA3 by a five step reaction sequence which involves selective protection of the 3-.beta.-hydroxyl group of GA3, preparation of the 13-methanesulfonyl derivative of the 3-acetate, hydrolysis of the acid chloride and reduction of the bridgehead-methanesulfonate, followed by hydrolysis of the resulting acetate. As described in U.S. Pat. No. 4,532,334, GA4 is obtained via Jones oxidation of a GA4/GA7 mixture, followed by Selectride.RTM. reduction. Another method for obtaining GA4 is selective degradation of GA7 from the mixture of GA4 and GA7, followed by isolation of GA4; but this method literally converts the GA7 into degradation products. None of these methods can provide GA4 and GA7 in large quantities efficiently. Alternatively, some authors have proposed the derivatization of gibberellins, including the formation of methyl esters and trimethylsilyl ethers, as a means of improving their detection and monitoring during analytical separation. Park, for example, in the Korean publication Han'guk Nonghwa Hakhoechi, 28(2):82-87 (1985), discloses the formation of such derivatives in connection with analysis by gas chromatography-mass spectroscopy. However, no suggestion is made that certain gibberellins may be selectively derivatized as is accomplished in the present invention, or that gibberellins so derivatized are more readily separated. Moreover, the above methods are for analytical rather than production use. Consequently, the need remains for an effective, large-scale separatory procedure.